Don’t ‘Supermassive’ Me: Black Holes Regulate Their Own Mass

Stellar-mass black holes, between 7 and 25 times the Sun’s mass, are  called  “micro-quasars” when they spawn powerful jets of particles and radiation, miniature versions of those seen in quasars. Stellar-mass black holes are on the small end of the scale opposite supermassive black holes, including those in quasars, which weigh millions to billions of times the mass of the Sun.

The micro-quasars’ jets may be part of a secret weapon for keeping their petite figures, according to new research.

NASA’s Chandra X-ray Observatory first spotted the interplay at a famous micro-quasar about 40,000 light years away in the constellation Aquila. This system, GRS 1915+105 (GRS 1915 for short), contains a black hole about 14 times the mass of the Sun that is feeding off material from a nearby companion star. As the material swirls toward the black hole, an accretion disk forms.

Two Harvard astronomers are revealing a newly discovered tug of war between the jets and hot winds from the material spiraling toward the black hole in what’s called an “accretion disk.” Both the jets and the hot wind eject matter from the stream that would otherwise help grow the black hole.

Chandra, with its spectrograph, has observed GRS 1915 eleven times since its launch in 1999. These studies reveal that the jet in GRS 1915 may be periodically choked off when a hot wind, seen in X-rays, is driven off the accretion disk around the black hole. The wind is believed to shut down the jet by depriving it of matter that would have otherwise fueled it. Conversely, once the wind dies down, the jet can re-emerge.

The accretion rate changes, but because of the interplay, the outflow rate remains constant.

“The black hole appears to be able to control how much matter it is or is not consuming at any given time,” said lead author Joseph Neilsen, a Harvard doctoral candidate. 

Self-regulation is a common topic when discussing supermassive black holes, but this is the first clear evidence for it in stellar-mass black holes.

Neilsen says it’s difficult to resist attributing a willfulness to the black hole’s behavior: “When you talk about regulation, it does imply some sort of self-control,” he said. ” We can see it’s happening, but it’s certainly not clear why. For now we just attribute it to some desire of the black hole.”

Although micro-quasars and quasars differ in mass by factors of millions, they should show a similarity in behavior when their very different physical scales are taken into account.

The timescale for changes in behavior of a black hole should vary in proportion to the mass. For example, an hour-long timescale for changes in GRS 1915 would correspond to about 10,000 years for a supermassive black hole that weighs a billion times the mass of the Sun.

“We cannot hope to explore at this level of detail in any single supermassive black hole system,” said co-author Julia Lee, a Harvard astronomer. “So, we can learn a tremendous amount about black holes by just studying stellar-mass black holes like this one.”

The new results appear in the March 26th issue of the journal Nature. 

ABOUT THE LEAD IMAGE: The optical and infrared image from the Digitized Sky Survey shows the crowded field around GRS 1915, located near the plane of our Galaxy. The inset shows a close-up of the Chandra image of GRS 1915, one of the brightest X-ray sources in the Milky Way galaxy. Credits: X-ray: NASA/CXC/Harvard/J. Neilsen et al. Optical: Palomar DSS2. A zooming video is available here.

Sources: NASA, the Nature study and an interview with Joseph Neilsen